Used by the golang test suite, and now broken out into a package, it asserts command output and includes full files.
For example, this script asserts that "cat" prints
hello world to stdout and nothing to
stderr (I think, if I'm reading it right!)
# hello world exec cat hello.text stdout 'hello world\n' ! stderr . -- hello.text -- hello world
found via https://encore.dev/blog/testscript-hidden-testing-gem ; I had seen it in the golang source before but never bothered to look at it in more detail.
A good model of creating tests that match your app.